docs: def downsampling(self, factor=10, first=3, switch=True, verbose=True):
          """
          rude downsampling of a `CMADataLogger` data file by `factor`,
          keeping also the first `first` entries. This function is a
          stump and subject to future changes. Return self.
  
          Arguments
          ---------
             - `factor` -- downsampling factor
             - `first` -- keep first `first` entries
             - `switch` -- switch the new logger to the downsampled logger
                  original_name+'down'
  
          Details
          -------
          ``self.name_prefix+'down'`` files are written
  
          Example
          -------
          ::
  
              import cma
              cma.downsampling()  # takes outcmaes* files
              cma.plot('outcmaesdown')
  
          """
          newprefix = self.name_prefix + 'down'
          for name in self.file_names:
              f = open(newprefix + name + '.dat', 'w')
              iline = 0
              cwritten = 0
              for line in open(self.name_prefix + name + '.dat'):
                  if iline < first or iline % factor == 0:
                      f.write(line)
                      cwritten += 1
                  iline += 1
              f.close()
              if verbose and iline > first:
                  print('%d' % (cwritten) + ' lines written in ' + newprefix + name + '.dat')
          if switch:
              self.name_prefix += 'down'
          return self